function show2(a:number,b:number,c?:number){
    c = c || 0.6;
    return (a + b) * c;
}

console.log(show2(1,2,3));

// 可选参数也可以使用附初始值
function get(a:number,b:number,c = 6){
    return (a + b) * c;
}
console.log(get(1,2));